1. Field of the Invention
The present invention generally relates to an image formation apparatus such as a facsimile apparatus.
2. Description of the Related Art
An image formation apparatus may use a laseroptics write device as a write means to form an image on a photosensitive body serving as an image holder where the formation of an image is done by use of multi-level laser powers corresponding to multi-level data values. The laser optical system uses a laser source that emits a laser beam modulated according to data, and scans the laser beam in a main scan direction by use of a polygon mirror after the laser beam passes through a collimator lens, an aperture, and a cylinder lens. The laser beam reflected by the polygon mirror is shone on the photosensitive body through an fθ lens, thereby writing data in the photosensitive body.
In the laser optical system, the power of laser light may differ depending on the position on the photosensitive body as shown in FIG. 3 due to the shading characteristic of the laser optical system even if the laser source emits the laser beam at a constant power along the main scan direction. The laser power on the photosensitive body is highest at the center of the main scan path, and drops towards the ends thereof. In order to achieve a constant laser power on the photosensitive body, the power of laser light should be corrected as shown in FIG. 4, thereby improving image quality. Such write device is disclosed in Japanese Patent Laid-open Application No. 11-286137.
The write device of Japanese Patent Laid-open Application No. 11-286137 corrects the laser power in order to achieve constant laser power on the photosensitive body. Because of sensitivity fluctuation of the photosensitive body, height fluctuation of the electric charger, etc., the correction of laser power based on fixed settings may not be effective always, resulting in a failure to sufficiently improve image quality in some cases.
When a development process is highly efficient, for example, the fluctuation of image density may be observed between the center and the ends of the main scan path. The shading correction is aimed at correcting the variation of laser power (becoming weaker toward the ends) along the main scan path on the photosensitive body when such variation is caused by the optical system. Since the shading correction is not perfect, charged potential on the photosensitive body may differ in the main scan direction as shown in FIG. 6. When the latent image is visualized on the photosensitive body, therefore, development potential differs in the main scan direction as shown in FIG. 7, creating varying degrees of development along the main scan path, thereby generating varying image density.
Accordingly, there is a need for an image formation apparatus which can improve image quality.
There is another need for an image formation apparatus which can improve image quality by correcting shading with respect to all colors in a balanced manner.
There is yet another need for an image formation apparatus which can improve image quality by applying optimum shading correction with respect to each dither patterns.
There is a further need for an image formation apparatus which can improve image quality by applying optimum shading correction with respect to each selected image-type mode.
There is a yet further need for an image formation apparatus for which additional software is kept minimum by simplifying computations.
There is a still further need for an image formation apparatus which can produce high-quality images having a proper gray balance.